Industrial power circuit protection is currently provided by thermal, magnetic, electromechanical and electronic circuit interruption devices tailored to respond to an inverse time-overcurrent function. A long-time delay before circuit interruption is provided for a low overload current value while a shorter time delay is provided for a higher overload current. Upon the occurrence of a short circuit current, the circuit must be interrupted instantaneously.
The thermal and magnetic trip units used within circuit breakers fairly approximate a time-overcurrent relationship which varies with the first power of time and with the second power of current (I.sup.2 t), whereas the induction discs used within electromechanical relays, for example, follow a polynomial relationship between current and time which approximates the 1.sup.2 t relation to a lesser extent. A good description of the operation of an electromechanical relay is found within a paper entitled "Digital Immerse Time Overcurrent Relay Using Counters" by M. Ramamoorty, which appeared in the ECI Journal, EL, 1980.
Time-overcurrent trip parameters for electronic circuit interruption devices of the so-called "analog" type use the delay characteristics of an electronic component such as a capacitor to determine the longtime and short-time delays before circuit interruption. U.S. Pat. No. 4,266,259 describes one such analog circuit interrupter.
Time-overcurrent trip parameters for electronic circuit interrupters of the so-called "digital" type are stored in electronic memory and are utilized within a microprocessor to provide long-time and short-time interruption. One example of a digital circuit interrupter is found within U.S. Pat. No. 4,672,501.
To provide interlock system coordination between interconnected electronic circuit interrupters, the downstream circuit interrupter signals the upstream circuit interrupter to delay the upstream circuit interrupter until the downstream circuit interrupter has reacted to clear an impending fault. One example of interlock function between electronic circuit interrupters is found in U.S. Pat. No. 4,468,714, which Patent is incorporated herein for purposes of reference.
When an electronic circuit interrupter is used as a main circuit interrupter within power distribution systems employing electromechanical relays or thermal and magnetic trip devices within branch circuits, some means of coordination is ordinarily required in order for the interruption device closest to the fault to interrupt the branch circuit before the electronic circuit interrupter interrupts the main circuit.
An early attempt to coordinate between electromechanical relays by using a digital electronic circuit interrupter is found within U.S. Pat. No. 4,275,445. A recent means for coordinating a digital circuit interrupter within circuits containing electric motors is described within U.S. patent application Ser. No. 256,357 filed Oct. 11, 1988 which Application is incorporated herein for purposes of reference.
Although such digital electronic circuit interrupters containing microprocessors programmed for specific time-overcurrent interruptions are known and commonly used, no such circuit interrupter is presently commercially available that can be tailored to fit a wide variety of time-overcurrent trip parameters such as found within the earlier described non-electronic interruption devices. One purpose of the instant invention, accordingly, is to describe a digital electronic circuit interrupter that can be custom-tailored in the factory or in the field to provide system coordination within specific power distribution systems containing both electronic as well as non-electronic circuit interruption devices.